.beta.-isophorone has great economic significance since it is an important synthetic structural element for the production of carotinoids, vitamins and pharmaceutical products. In particular, .beta.-isophorone is required as a precursor for ketoisophorone (2,6,6-trimethylcyclohex-2-ene-1,4-dione) and trimethylhydroquinone and therewith for the production of vitamin E. In addition, it is pivotably used in syntheses for odorous substances and natural compounds such as astaxanthine and abscisic acid and derivatives.
The production of isophorone is carried out by means of acetone trimerization under condensation of the C.sub.3 structural elements. The primarily formed isomer is .alpha.-isophorone since it has, in contrast to the .beta. isomer, a double bond conjugated to the keto function. For this reason the thermodynamic equilibrium is on the side of the .alpha.-isophorone; the .beta. concentration is only approximately 1-2% and the adjustment of equilibrium takes place very slowly.
Although there are basically two different methods of preparation for arriving at ketoisophorone, namely, the direct oxidation of .alpha.-isophorone (.alpha.-IP).fwdarw.ketoisophorone (KIP) and the indirect route via the isomerization .alpha.-isophorone.fwdarw..beta.-isophorone (.beta.-IP) in a primary step and subsequent oxidation of the .beta.-isophorone.fwdarw.ketoisophorone, the latter process is clearly advantageous. Scheme 1 presents these considerations for ketoisophorone synthesis in a clear manner. ##STR1##
Numerous methods for the isomerization of .alpha.-IP have been described in the course of time which, however, have significant disadvantages. Viewpoints such as consumption of chemicals, poor space/time yields and problems in the workup have prevented, up to the present, a practical processing reaction on a large scale.
A number of publications are concerned with the isomerization in the liquid phase. The more pertinent state of the art is represented by the following publications:
D1=A. Heymes et al., Recherches 1971, 18, 104 PA0 D2=FR-A-1,446,246 PA0 D3=DE-OS-24 57 157 PA0 D4=U.S. Pat. No. 4,005,145 PA0 D5=EP-A-0,312,735 PA0 D6=JP 87-33019 corresp. to HEI-1-175954 of Jul. 12, 1989.
D1 discloses the isomerization of .alpha.-IP to .beta.-IP with stoichiometric amounts of MeMgX (Me=methyl, X=halogen-) Grignard compound. 73% .beta.-IP is obtained with evolution of methane in the presence of catalytic amounts of FeCl.sub.3.
D2 relates to the isomerization of .alpha.-IP to .beta.-IP in the presence of catalytic amounts of p-toluene sulfonic acid and generally aromatic sulfonic acids, especially aniline sulfonic acid. The amount of the catalyst used is 0.1-0.2 % relative to the .alpha.-IP used. However, a low degree of conversion and a high accumulation of byproducts prevent an industrial application of the method of D2.
According to D3, the preparation of .beta.-IP takes place by means of boiling .alpha.-IP for several hours in triethanol amine, fractionation, washing the distillate with tartaric acid and sodium chloride solution. The consumption of chemicals is also considerable here.
In D4, acids with a pK=2-5 and a higher boiling point than .beta.-IP (boiling point .beta.-IP=186.degree. C./760 mm Hg) are used as catalyst. The following are named:
Aliphatic and aromatic amino acids, adipic acid, p-methylbenzoic acid, 4-nitro-m-methylbenzoic acid, 4-hydroxybenzoic acid, 3,4,5-trimethoxybenzoic acid, vanillic acid, 4-trifluoromethylbenzoic acid, 3-hydroxy-4-nitrobenzoic acid and cyclohexane carboxylic acid and derivatives. The amount of catalyst used is 0.1-20 molar percent. The yield of .beta.-IP (relative to .alpha.-IP used) is 74.5%.
At a rate of decrease of 11 ml/h .beta.-IP and a simultaneous amount added of approximately 0.5 kg .alpha.-IP, the space-time yield and the production of .beta.-IP is Y=0.218 kg .beta.-IP/kg.sub.cat /h and is thus too low to find industrial application. These values correspond to a space-time yield of Y.sub.s-t =0.015 l.sub..beta.-IP /h/l.sub.solution.
A similar principle is followed in D5. Acetyl acetonates of transitional metals are used as .pi. bond displacement catalysts. Even Al (acac) displays catalytic activity. The use of the catalyst takes place in 0.01-10% by weight related to the starting weight of .alpha.-IP. Metallic catalysts of groups IVb (Ti/Zr/Hf), Vb (V/Nb/Ta), VIb (Cr, Mo, W), VIIb (Mn/Tc/Re), the entire group VIII and aluminum are patented. The primarily obtained distillate has a .beta.-IP content of 94%, a further Vigreux distillation enriches the .beta.-IP content to 99%. This result corresponds, relative to amount of catalyst used and the time, to a yield of Y--9.4 liters .beta.-IP per kilogram catalyst per hour. This corresponds, relative to the educt solution used, to a yield of Y.sub.s-t =0.0376 l.sub..beta.-IP /h/l.sub.solution.
According to D6, the isomerization takes place in the liquid phase at temperatures around 200.degree. C. Silica gels with or without the addition of alkyl-substituted imidazolines of the following formula are used as catalyst. ##STR2##
Typical experimental conditions: 300 g .alpha.-IP and 25.7 g SiO.sub.2 are distilled 52 h in the presence of refined steel; 230 g .beta.-IP (=76.6% yield) result with 99.9% purity. This result corresponds, relative to amount of catalyst used and the time, to a yield of Y=0.174 liters .beta.-IP per liter catalyst per hour.
Moreover, the procedure described is unfavorable and the absolute production of .beta.-IP low. The performance of isomerization and one-step distillation of the .beta.-IP in one step is especially disadvantageous. It can be demonstrated that the re-isomerization of .beta.-IP to .alpha.-IP occurs to a considerable extent on account of the high reaction temperature in the distillation apparatus.